Sleep loss is extremely prevalent during adolescence. The causes of sleep restriction in adolescence are the result of brain/behavior/social context interactions in sleep-wake and circadian regulation. Biological changes at puberty create a natural tendency to prefer staying up late as well as increased sleepiness. However the key sleep behaviors (patterns of staying up late and erratic sleep/wake schedules) are largely an interaction between these biologic tendencies and the social environment (i.e., effects of artificial light, stimulating social media such as texting and the internet, and other highly rewarding activities combined with early school start times). Moreover, these interactions are conspiring to rob youth of sleep at a time of critical maturational changes in physical, social, and affective development. Sleep disruption in adolescence may initiate a negative cascade of impaired reward and emotion processing; affective and behavioral dysregulation; further sleep problems; and ultimately, clinical depression and substance abuse. The broad goal of this research is to elucidate potential biopsychosocial mechanisms linking sleep problems with the development of depression and substance abuse in adolescence. Understanding the mechanisms by which sleep loss contributes to vulnerabilities for affective dysfunction and behavioral and mental health problems triggered during adolescence can provide leverage for developing more effective early interventions, including both clinical and social policy-level efforts aimed at the social environment (i.e., school start times, parental and youth education programs, and behavioral interventions). As a first step to address this goal, the proposed study will evaluate the effects of well-controlled laboratory manipulations of sleep duration and social context on activity in neural systems supporting reward and emotion regulation, and on related affective behaviors. Our specific aims are: (1) to investigate the impact of transient sleep restriction and peer social context on reward and risk-taking; and (2) to investigate the impact of transient sleep restriction and peer social context on emotional reactivity/ regulation. Using a within-subjects crossover design, youth in middle adolescence will be studied under two experimental conditions: (1) sleep restriction (two nights of 4 hours time in bed) and (2) sleep extension (two nights of 10 hours time in bed). Polysomnographically-monitored sleep conditions will be followed by functional magnetic resonance imaging (fMRI) and behavioral testing the next day. We will employ validated reward and emotion processing neuroimaging tasks to examine cortical-subcortical activation in response to appetitive and aversive stimuli. Participants will be studied in pairs of friends, enabling us to examine the influence of the peer social context on affective functioning. We will also examine neural and behavioral responses to socially- relevant stimuli.